Electronic strobe flash systems mounted on or in a camera and involving the rapid discharge of a high voltage through a flash lamp synchronized with the exposure of an image frame are well-known in the photographic art. For a flash exposure of a scene, it is desirable to illuminate the scene indirectly so that it appears to be illuminated from above in a way characteristic of natural lighting and to illuminate the scene directly to soften dark shadows caused by obliquely directed light. The indirect illumination is commonly known as bounce flash, since the light is typically directed upward and bounced off a reflective ceiling and onto the scene, although it is also known to bounce the flash off a vertical wall or panel to provide side illumination. Flash light aimed directly at the scene is referred to as direct illumination or fill-in flash. Where natural illumination from above is available, such as in daylight outdoor scenes, direct or fill-in flash may be employed to soften shadows falling on the subject to be photographed. Conversely, in indoor scenes, where illumination from above is not available or does not possess natural light spectral characteristics, it is desirable to provide indirect illumination by bouncing flash light off a ceiling and onto the scene or subject to be photographed. In such situations, it is also desirable to provide direct illumination so as to again fill in shadows created by the downwardly directed bounce flash illumination. The combination of bounce and fill in flash illumination minimizes the undesirable effects of direct illumination alone, including harsh shadows, red eye, specular reflections, and high contrast or loss of tonal detail depending on the distance of subjects in the image. Unfortunately, indirect flash illumination requires much more energy than direct illumination due to light intensity loss by absorption and scattering by the ceiling or wall surface.
U.S. Pat. No. 4,242,616 describes a photographic flash apparatus for providing both fill-in and bounce flash illumination provided by a direct illumination flash bulb 17 and an indirect illumination flash bulb 19, which are simultaneously charged from a single battery 20 and a discharge capacitor 23. The high voltage capacitor 23 is discharged simultaneously through the flash tubes 17 and 19 synchronously with the operation of the camera shutter. By configuring the direct and indirect flash tubes differently and choosing appropriate circuit components, bounce flash illumination provided by the indirect flash tube 19 exceeds the fill-in illumination provided by the direct flash bulb 17. The amount of illumination provided is measured by a photo detector aimed at the scene, and both fill-in and bounce flash light is terminated simultaneously when a desired total reflection of light from the scene is achieved. A ratio of indirect to direct illumination of about 75:25 is sought in the operating example of the circuit described in the '616 patent.
U.S. Pat. No. 4,384,238 discloses an electronic strobe flash apparatus for bounce and fill in flash illumination of a scene also having direct and indirect flash tubes coupled in parallel to a charging capacitor, battery, DC/DC converter, and control circuit and to separate quenching circuits for separately halting discharge through each of the flash tubes. In the flash apparatus of the '238 patent, the control circuit is switched into operation synchronously with the camera shutter release and first causes the indirect flash lamp to discharge and emit bounce flash illumination on the scene. During a 40 microsecond period, a photo detector circuit measures the light reflected from the scene and, if the reflected light falls below a certain threshold indicating the absence of a suitable reflective ceiling surface, the bounce flash illumination is quenched and fill-in flash illumination is simultaneously commenced. If, however, the bounce flash illumination reflected from the scene is sufficiently high, then bounce flash illumination continues until the reflected light reaches a second threshold, whereupon bounce flash illumination is quenched and fill-in illumination is commenced. The photo detector and control circuit continues to monitor the accumulated reflected light and quenches fill-in illumination when the total measured illumination reaches a further threshold related to the film type and shutter speed. The user may override the control circuit by disabling bounce flash illumination when there is sufficient overhead illumination or when no ceiling is present.
A further U.S. Pat. No. 5,136,312 to Weaver et al, assigned to the assignee of the present invention, describes a direct and indirect flash illumination system wherein the presence of a suitable reflective ceiling surface is detected by light reflected therefrom is employed to control the ratio of indirect and direct illumination and, in conjunction with light reflected from the scene, total illumination. In one embodiment, an active radiation emitter transmits IR radiation upward in a manner such that a portion of the radiation is reflected back to a photo detector which provides a first signal containing information relating to the distance between the surface and the flash system as well as the quality of the reflective surface. A control system responsive to the signal causes both bounce and fill-in flash illumination when the indirect reflecting surface is within a predetermined distance and causes operation of only fill-in or direct flash illumination when the indirect reflecting surface is not within the predetermined distance. The first signal and a further signal developed by a photodetector aimed at the scene are employed in controlling total illumination by separately quenching the direct and indirect illumination.
In an alternative embodiment, a photo detector aimed upward is employed in a passive mode to detect bounce flash illumination reflected back from the ceiling, if present, in order to provide the first signal under the circumstances previously described. Thus, the '312 patent discloses a system which determines the presence or absence of a suitable reflective ceiling by measuring light reflected therefrom and controls bounce flash illumination in dependence thereon. Variable flash output and aperture selection can be combined in a single system to match the optical depth of field with the depth of illumination provided by the bounce and/or fill-in flash illumination. A quick recycle mode is also disclosed wherein bounce flash illumination is suppressed. In the various disclosed embodiments, when bounce flash illumination is allowed, it commences simultaneously with fill-in flash illumination, and the illumination from both the direct and indirect flash tubes is quenched either simultaneously or in an order dependent on the detected light returning from the scene in accordance with known exposure control algorithms.
In a further U.S. Pat. No. 5,055,865 to Fujino et al, a pair of separately controlled direct and indirect flash apparatus are disclosed which may or may not be incorporated into the camera body wherein bounce and fill-in illumination are provided of a scene under a number of operating algorithms. Preferably, the fill-in illumination is provided by a direct flash lamp, battery, charging circuit and control circuit, all incorporated in the camera body, and bounce illumination is provided by an indirect flash apparatus having a self-contained battery, charging capacitor and micro-computer based control system which may be mounted to hot-shoe of the camera body as if the camera had no internal flash apparatus. The various modes of operation include a charging subroutine illustrated in FIG. 7 where it appears that charging of the external and internal flash apparatus high voltage capacitors is triggered simultaneously and the priority of direct and indirect flash illumination may depend on which capacitor charges up first. The speed of charging the high voltage capacitor of the internal flash apparatus may be affected by prioritized allocation of current drawn from the battery for powering other camera functions, such as automatic focus and motorized film advance as shown in FIG. 17.
The "bounce surface" from which a flash of light can be bounced to illuminate a subject either below or beside the surface, such as a ceiling or a wall, typically varies considerably in reflectivity. Inherently, a large amount of the flash light directed at the bounce surface is either absorbed or reflected elsewhere than on the scene intended to be illuminated, particularly as the distance between the indirect flash lamp and the bounce surface increases. Due to the inherent dissipation of the bounce illumination, the indirect flash units have been designed to deliver two or three times as much illumination as the direct illumination flash lamps, as described above. Thus, the prior patents described above have emphasized the desirability of providing the bounce flash illumination first or at the same time as the fill-in flash illumination of the scene. Since the energy requirements for the bounce flash illumination well exceed the requirements for direct flash illumination, a problem arises in the time delay it takes to charge up a single high voltage capacitor to a voltage sufficient to deliver the required bounce flash illumination first. It would be desirable to decrease the time between successive exposures with the fill in flash and to avoid unnecessary charging of the high voltage capacitor for the bounce flash.